Carbohydrate alkylcarbamates are applied as surface-active substances. Their preparation is disclosed in, inter alia, Maunier et al. (1997) Carbohydrate Res. 299, 49-57, which disclosed tensio-active properties of several 6-aminocarbonyl derivatives of methyl a-D-glucopyranoside and D-glucose and compared them with those of the 6-aminocarbonyl derivatives of the urethane named methyl 6-0-(N-heptylcarbamoyl)-a-D-glucopyranoside. Lesiak et al. (1980) J. Prakt. Chem. 222, 877-883, disclosed the use of 1-methyl-2,4-bis(isocyanate)benzene (commonly named tolylene-2,4-diisocyate; in short 2,4-TDI) for the synthesis of urethanes of glucose and sucrose. First 2,4-TDI was reacted with various aliphatic long chain alcohols or terpene alcohols in a molar ratio of 1:1. The reaction essentially occurred with the isocyanate group at position four, and the urethanemono-isocyanates obtained were then reacted with glucose or sucrose, respectively, to yield the corresponding di-urethanes, which presented moderate tensio-activity. The synthesis of several sucrose N-n-alkylurethanes and their tensio-active properties have been disclosed inter alia by Bertsch et al. (1960) J. Prakt. Chem. JL1, 108, and by Gerhardt (1967) Abh. Dtsch. Akad. Wiss. Berlin, Kl. Chem., Geol. Biol. 1966(6), 24-32.
The urethanes have been prepared by the reaction of sucrose with the selected n-alkyl isocyanate (H. Bertsch et al. o.c.) and by transformation of sucrose with potassium cyanate and an n-alkylhalogenide in dimethyl formamide (W. Gerhardt, o.c.).
Chauvin et al. (1993) J. Org. Chem. 58, 2291-2295 shows methods wherein N-alkyl sucrose carbamates are treated with N-butanol after partial removal of DMF, concentrated and purified by column chromatography.
Alkyl urethanes obtained by the reaction of n-alkyl isocyanates with fructans are disclosed in patent application EP1086197/WO9964549. These alkyl urethanes exhibit excellent surface-active properties. Surface-active substances are also obtained on the basis of the reaction of n-alkyl isocyanates with starches in EP application EP1237932A1.
In the abovementioned prior art, the synthesis and the reactivity of carbon hydrates with n-alkyl isocyanates are extensively described, as are the properties of the end products obtained as surface-active substances. In each reaction the carbohydrate is dissolved in an aprotic reaction solvent (further also called first solvent) such as, inter alia, DMF (dimethyl formamide), DMAC (dimethyl acetamide), DMSO (dimethyl sulfoxide), NMP (N-methyl-pyrrolidone) and NEP (N-ethyl-pyrrolidone), since water is not suitable in view of its own reactivity with the n-alkyl isocyanates.
However, the isolation of the carbohydrates that have reacted with the n-alkyl isocyanates to form the corresponding carbamates always takes place via a precipitation reaction, optionally preceded by partial evaporation of the reaction solvent.
For isolation and precipitation of the carbohydrate alkylcarbamates, use is made of additional solvents of the type of alcohols, ketones, ethers and esters. After precipitation, techniques such as decantation, filtration or centrifugation, followed by multiple washes with the same additional solvent, have to be used to obtain an end product containing minimal amounts of reaction solvent.
The reason for this is that carbohydrate alkylcarbamates are used in applications such as cosmetics, paints, coatings, wax emulsions, detergents, etc., in which minimal concentrations of the reaction solvents are allowed because of their toxic nature.
The abovementioned purification methods therefore have major drawbacks. In the first place, in order to achieve an acceptable quality in terms of residual reaction solvent, the quantities of additional solvent needed are large in proportion to the desired end product. The ratio of the former to the latter is at least five to ten times, and more likely twenty to thirty times. Moreover, the additional solvents used often are highly flammable and must be recovered after isolation of the carbohydrate alkylcarbamate by distillation of the reaction solvent to limit the environmental impact. In view of the large quantities of solvents to be separated, the impact on the costs of the end product is such as to render commercialisation impossible.
Moreover, no quantitative yields can be achieved with this isolation method since low-molecular carbohydrate alkylcarbamate still retain a certain solubility in the mixture of reaction and additional solvent, and as such are not co-precipitated. Typical yields of purified end product obtained range from 55 to 95%, depending on the type of carbohydrate alkylcarbamate. However, a 5% yield loss has a significant impact on the economic scalability and also means an impact on the environment because of the non-recoverability.
In the third place, it is very difficult to control the precipitation, that is to say, control of the physical shape of the precipitate obtained (granule size) is very hard and unpredictable, making it extremely difficult to use current techniques such as decantation, filtration and centrifugation, since the granule size of the precipitate determines the speed and the efficiency of these separation techniques.
For this reason, there is a need within the field for more efficient ways to remove the solvents.